This invention is directed to a process for producing polyarylates using a diluent which can be easily separated from the reaction products
Polyarylates are polyesters derived from a dihydric phenol, particularly 2,2-bis(4-hydroxyphenyl)propane, also identified as bisphenol-A, and an aromatic dicarboxylic acid, particularly mixtures of terephthalic and isophthalic acids. These polyarylates are high temperature, high performance thermoplastic polymers with a good combination of thermal and mechanical properties. They also have good processability which allows them to be molded into a variety of articles.
Many processes have been described in the literature for the preparation of polyarylates. One such process is the diacetate process. In the diacetate process, a dihydric phenol is converted to its diacetate, which is then reacted with an aromatic dicarboxylic acid(s) to form the polyarylate.
However, the polyarylates produced by the diacetate process have a tendency to be colored which could make them unsuitable in some end use applications. Thus, there is a desire to improve the color of the polyarylate for such end use applications.
In addition, there are other problems in the diacetate process for producing polyarylates which must be economically and practically solved in order to have a viable, economically attractive process. One problem occurs when the reaction takes place in the molten state. In such molten reaction, sublimation of the diacid occurs. This disrupts the stoichiometry of the reaction and the polyarylate produced is not of acceptable molecular weight. Another problem when a diester derivative of a dihydric phenol is reacted with an aromatic dicarboxylic acid in the molten state is that the viscosity of the system increases dramatically towards the end of the reaction and therefor the reaction becomes diffusion controlled (the molecules are not close enough to insure rapid reaction) rather than kinetically controlled. Also, the polymer product is difficult to handle (i.e., removal from the reactor) due to this high viscosity.
Yet another problem in the production of polyarylates by the diacetate process is that a carboxylic acid is a by-product of the reaction. In order to provide an efficient, economical process and a high molecular weight polyarylate, the acid, for example, the acetic acid has to be conveniently and efficiently removed.
U.S. Pat. No. 4,294,956 discloses that the reaction of the diester derivative of a dihydric phenol with the aromatic dicarboxylic acid can be carried out in a diphenyl ether solvent which is present in amounts of from about 10 to about 60, based on the weight of the polyarylate produced. The diphenyl ether compound may be substituted. These substituents are selected from alkyl groups, chlorine, bromine or any substituent which does not interfere with the polyarylate forming reaction or the reaction forming the diester derivative of the dihydric phenol. Additionally, the diphenyl ether compound may be used with up to 50 weight percent of other compounds, such as various biphenyls or any other compounds which do not interfere with the polyarylate forming reaction or the reaction forming the diester derivative of the dihydric phenol. A preferred solvent comprises a eutectic mixture of diphenyl oxide and biphenyl, which solvent is a liquid at room temperature which is marketed under the name Dowtherm A from Dow Chemical Corp.
The utilization of from about 10 to about 60 percent of a diphenyl ether compound in the diacetate process reduces sublimation of the aromatic dicarboxylic acid; thus producing polyarylates of acceptable molecular weight. Also, the diphenyl ether compound provides for better removal of the acetic acid by-product. Further, an additional benefit is that the viscosity of the system is decreased. This decrease in viscosity provides a faster reaction time since better mixing of the reactants occurs which allows the reaction to proceed under kinetic control. Additionally, reaction times are relatively short so that a polyarylate is produced generally in less than 10 hours at the reaction temperatures and the polyarylates produced possess lighter color, as compared to those utilizing longer reaction times.
While the formation of polyarylates by the diacetate process has worked sufficiently well utilizing a diphenyl ether diluent such as disclosed in U.S Pat. No. 4,294,956, there still remains the problem of separating the diphenyl ether from the acetic acid byproduct. Thus, the diphenyl ether and acetic acid form a solution necessitating additional process steps to recover the diluent for return to the polymerization process. Moreover, polyarylate color still remains a problem even when using the diluent in polymer formation.
Other diluents or solvents have been suggested for producing polyarylates by the diacetate process. For example, U.S. Pat. No. 4,374,239 discloses polymerizing the diester of a dihydric phenol with an aromatic dicarboxylic acid in the presence of at least one halogenated and/or etherated substituted aromatic or heteroaromatic compound. Examples include 1,2, 4-trichlorobenzene, 1,4-dimethoxy benzene and o-dichlorobenzene. Many other examples are cited in the patent disclosure.
U.S. Pat. No. 4,533,720 discloses forming polyarylates by the diacetate process utilizing a gamma-butyrolactone as a diluent. The patent discloses that this diluent reduces sublimation of reactants resulting in higher molecular weight polymers.
There is still a need to provide a useful diluent for preparing polyarylates by the diacetate process which will have the advantages of the prior art in reducing sublimation of the monomeric components and thus produce polyarylates of acceptable molecular weight and color and to reduce the viscosity of the polymerization system. There is also a need to provide a diluent which will facilitate better removal of the acetic acid byproduct and at the same time be easily separated from the acetic acid byproduct for recycling.